Radio frequency identification (RFID) systems typically use one or more reader antennas to send electromagnetic carrier waves encoded with digital signals to items containing or equipped with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In the case of passive RFID systems, an RFID tag is powered by the electromagnetic carrier wave. Once powered, the passive tag interprets the radio frequency (RF) signals and provides an appropriate response by creating a timed, intermittent disturbance in the electromagnetic carrier wave. These disturbances, which encode the tag response, are sensed by the reader through one of the reader's antennas, thus completing the reader-tag communications loop. In the case of active or semi-passive RFID systems the tag contains its own power source. In the active RFID systems, the power is used for communications with the reader by creating tag's own carrier wave and encoded RF signals; while in the semi-passive RFID systems, tag's power supply is solely used for powering the tag's IC, not for communication.
In most item-level RFID applications, each item is tagged with an associated unique ID. Item-level RFID can empower users to precisely track inventory and location of tagged items in real time or near real time. In many applications large quantities of tagged items are placed in a crowded environment, and often the passive RFID tags on or in those items have very small form factors. In order to realize item-level visibility, the reader antennas have to be brought into close proximity with these passive RFID tags due to the tags'short working range. However, traditional antennas are designed for far-field applications, not for near-field applications. In applying traditional antenna designs to near-field RFID applications we face several inherent problems: 1) some antenna parameters are no longer valid to evaluate the antenna performance, such as directivity; 2) there may exist dead zones in the proximity of the antenna, i.e., tagged items that fall in these zones will not be detected; 3) the effective coverage may not be large enough to cover a specified area. This last problem can be solved by implementing an antenna array. However, this increases the system cost, complexity, and response time.